Common mode noise (CMN) is the noise formed when differential signals do not perfectly compliment to each other. Common mode voltage (Vcm) of differential signals having voltages Vdp and Vdn is defined as Vcm=(Vdp+Vdn)/k, where ‘k’ is a constant (e.g., k=2). CMN is generated from differential signals due to phase misalignment of the differential signals, rise and fall time mismatch, amplitude mismatch, mismatch in skew between differential signals and timing mismatch due to non-ideal differential signaling from an IO (input-output) buffer. CMN is also generated by non-idealities or mismatch introduced due to platform component (e.g., differential trace length; via mismatch; impedance mismatch, etc).
CMN is currently controlled using an on-board Common-Mode Choke (CMC). But adding CMC increases manufacturing cost which may not be a viable solution for producing low cost products. Adding CMC also causes CMN to differential gain if CMN is not cancelled at the platform or board level which results in overshoot, undershoot, and ringing to the differential signals. Overshoot and undershoot voltage caused by CMN may be so high that they may cause reliability issues for the devices.
CMN is also currently controlled, during the design and layout phases of the IO buffer, by controlling signal timing skew, control of signal rise/fall-time, and signal amplitude skew. However, such an approach is static and does not account for non-idealities of the actual design when being used in a real product.